Pyrolysis temperature affects phosphorus transformation in biochar: Chemical fractionation and 31P NMR analysis

Phosphorus (P) recycling or reuse by pyrolyzing crop residue has recently elicited increased research interest. However, the effects of feedstock and pyrolysis conditions on P species have not been fully understood. Such knowledge is important in identifying the agronomic and environmental uses of biochar. Residues of three main Chinese agricultural crops and the biochars (produced at 300 degrees C-600 degrees C) derived from these crops were used to determine P transformations during pyrolysis. Hedley sequential fractionation and P-31 NMR analyses were used in the investigation. Our results showed that P transformation in biochar was significantly affected by pyrolysis temperature regardless of feedstock (Wheat straw, maize straw and peanut husk). Pyrolysis treatment transformed water soluble P into a labile (NaHCO3-Pi) or semi-labile pool (NaOH-Pi) and into a stable pool (Dil. HCl P and residual-P). At the same time, organic P was transformed into inorganic P fractions which was identified by the rapid decomposition of organic P detected with solution 31P NMR. The P transformation during pyrolysis process suggested more stable P was formed at a higher pyrolysis temperature. This result was also evidenced by the presence of less soluble or stable P species, such as such as poly-P, crandallite (CaAl3(OH)(5)(PO4)(2)) and Wavellite (Al-3(OH)(3)(PO4)(2) . 5H(2)O), as detected by solid-state P-31 NMR in biochars formed at a higher pyrolysis temperature. Furthermore, a significant proportion of less soluble pyrophosphate was identified by solution (2%-35%) and solid-state (8%-53%) P-31 NMR, which was also responsible for the stable P forms at higher pyrolysis temperature although their solubility or stability requires further investigation. Results suggested that a relatively lower pyrolysis temperature retains P availability regardless of feedstock during pyrolysis process. (C) 2016 Elsevier B.V. All rights reserved.